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Power Calculations Svati Shah Analyst Meeting October 4, 2007

Power Calculations Svati Shah Analyst Meeting October 4, 2007. What is Power?. ‘a measure of how likely the study is to produce a statistically significant result for a difference between groups of a given magnitude’. Bowling, 1997; P.149

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Power Calculations Svati Shah Analyst Meeting October 4, 2007

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  1. Power Calculations Svati ShahAnalyst Meeting October 4, 2007

  2. What is Power? • ‘a measure of how likely the study is to produce a statistically significant result for a difference between groups of a given magnitude’. Bowling, 1997; P.149 • Power is ‘the probability of correctly rejecting a false null hypothesis’. Howell, 1987: P.204. • If you report power as .8 this means that you have an 80% probability of detecting a difference (or ‘correctly rejecting a false null hypothesis’ ie accepting H1).

  3. H0:Person A is not guilty H1:Person A is guilty – send him to jail In reality… H0 is true H1 is true β 1 - α H0 is true Type-2 error We decide… 1 - β α H1 is true Power Type-1 error Power: probability of declaring that something is true when in reality it is true.

  4. H0:There is NO linkage between a marker and a trait H1:There is linkage between a marker and a trait I decide H1 is true I decide H0 is true x Threshold Power (1 – β) Type-1 error (α) High High Too low Low Low Too high

  5. What Determines Power? • Magnitude of difference between groups (often known as effect size). • Type of statistical test (parametric tests more powerful) • Design (within subjects, more powerful) • One tailed or 2 tailed test • Sample size • MAF

  6. Calculating Effect Size • Three ways to establish effect size: • Look at previous research and calculate effect size from that • From pilot work • Estimate what you would like to find/what would be clinically significant

  7. Why is it important to estimate power? To determine whether the study you’re designing/analyzing can in fact localize the QTL you’re looking for. Study design and interpretation of results. You’ll need to do it for most grant applications. When and how should I estimate power? When? How? Study design stage Theoretically, empirically Analysis stage Empirically

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